The invention relates to a contact unit adapted to be embedded in a mass of electrically conductive sheet or casting on one's skin for applying electric current for therapeutic or beauty purposes, the current being supplied by a suitable voltage generator, and the contact unit ensuring the electrical connection between the generator lead and the electrically conductive casting composition.
U.S. Pat. No. 4,317,457 discloses a cutaneous electrode or electrically conductive casting for treating a person's skin. The casting is prepared from a plastic binder with an electrically conductive additive. When excess water is added to the binder which is typically a dental quality plaster, a settable material is formed. The additive is a salt dissolved in the water, typically calcium chloride. In the foregoing U.S. patent at least one metal contact is embedded in the casting composition.
The electrical resistivity of the electrically conductive casting composition is substantially greater than that of a metal. A surface resistivity similar to that of one's skin must be sought in order that the skin be subjected to comparable current densities over the entire surface of the electrically conductive casting composition which means that the voltage drops between the point where current is applied to the casting and any other point along the surface of the casting is substantially the same in the casting and in the subjacent cutaneous tissue.
It has been found that beneath the zones where the metal contacts are incorporated in the casting, the current densities were greater than the average current densities along the casting. Indeed, owing to the relatively low resistivities, the metal contacts are substantially equipotential and the lines of current flow entering the casting composition are normal to the contact surface. The distance between the contact and the skin inside the casting is reduced so that the current is concentrated. This limits the effective current for treatment and increases the risk of burning the skin if the thickness of the casting is insufficient beneath the metal contact. The positioning of the embedded metal contact in the casting requires care and training and even then, the current density must be adjusted with caution.